To determine the three-dimensional structure of biological macromolecules (proteins, enzymes, and nucleic acids) using the method of single-crystal x-ray diffraction. Major emphasis is given to macromolecular complexes and complex interactions between macromolecules. Systems studied include DNA replication, the energy-dependent destruction of proteins, and proteins implicated in autoimmune diseases. Structural studies of retroviral proteins and enzymes are also underway to understand the mechanism of retroviral entry and integration. Knowledge of a macromolecule's three-dimensional structure is vital to understanding its biological function. Biological macromolecules often function by arranging themselves into larger assemblies in the cell. The chosen model systems for DNA replication and energy-dependent proteolysis are processes indispensable to all living cells. Work to determine structures of retroviral target enzymes and autoimmune proteins is essential for future structure-based drug development and other potential therapeutic strategies. We have determined the high resolution crystal structure of the core domain of GP41 from Simian Immunodeficiency Virus using a combination heavy-atom and molecular replacement methods. The trimeric protein structure consists of three interior helices packed in a parallel coiled-coil arrangement, with smaller outer helices packed in an antiparallel direction. The structure confirms models of GP41 derived from crystal structures of smaller, discontinuous peptide fragments. The important interconnecting loop regions are evidently disordered in our crystal form and cannot be modelled at present. Work is underway to define these regions and their role in binding retroviral GP120 protein and in effecting viral attachment and membrane fusion. Well-diffracting crystals of several other proteins have been obtained and crystallographic structure determinations are underway. These include a human form of the CLP-P protease, multidomain forms of a retroviral integrase, a helicase accessory protein involved in DNA replication, and a truncated form of a human autoantigen.